The present invention is directed to a sewage wastewater treatment system comprising a septic tank and aerobic filter, and in particular to an aerobic filter that utilizes a textile filter media.
A septic tank typically provides primary treatment for domestic wastewater. In a conventionally operated septic tank, raw untreated sewage wastewater having a significant concentration of waste solids is introduced into the tank from an adjacent building. In the septic tank, solids separate from the liquid portion of the sewage. Solids having a lower density than the liquid move to the top of the liquid to form a scum layer, and solids having a higher density than the liquid sink to the bottom of the tank to form a sludge layer, resulting in a relatively clear liquid layer between the scum and the sludge. This liquid portion of the wastewater, which exits the discharge end of the tank by means of gravity, a pump, or a siphon, is the septic tank effluent.
The quality of the septic tank effluent primarily determines its subsequent disposition, including the size and kind of any required secondary waste treatment facilities. Such effluent quality is generally measured by the biochemical oxygen demand (BOD), total suspended solids (TSS), and total nitrogen present in the effluent.
Nitrogen in raw untreated wastewater is primarily organic nitrogen combined in proteinaceous material and urea. Decomposition of the organic material by bacteria present in the anaerobic environment of the septic tank changes the organic nitrogen to ammonia nitrogen. Thus, in conventionally treated septic tank effluent, nitrogen is present primarily as ammonia nitrogen.
Secondary treatment of septic tank effluent is typically an aerobic treatment. In addition to its reduction of BOD and TSS, the aerobic environment of secondary treatment causes microorganisms (bacteria) to oxidize ammonia nitrogen to nitrate nitrogen, a process known as nitrification. Thus, in a conventional system, nitrogen in the secondary treatment effluent is primarily nitrate nitrogen. The secondary treatment effluent is either discharged directly from the system, or undergoes at least partial recirculation through a recirculation tank as shown in U.S. Pat. No. 5,480,561.
Several different types of aerobic filters, having different filter media and configurations, have been utilized to provide the secondary treatment. A conventional aerobic filter is disclosed in Ball et al., U.S. Pat. No. 5,531,894. Ball et al. describe the filter media as either coarse sand, or synthetic media of, for example, PVC corrugated sheets or lengths of pipe, or randomly packed pieces of polymeric material. The filter media provides a support for the microorganisms to form a fixed film biological growth.
It has been desired to use textile material for the filter medium in the aerobic filter. Textile sheets provide a potential advantage over sand since they are much lighter and provide more surface area for treatment. Thus, secondary filters which use textile material have the potential advantage of smaller, lighter weight secondary filters.
Hall, U.S. Pat. No. 3,301,401, discloses an aerobic filter comprised of a plurality of hanging sheets. A plurality of deflectors or baffles are positioned adjacent to the upper portions of the sheets. A liquid distributor is positioned above the sheets and delivers a spray onto the deflectors and the sheets.
Yet another prior art aerobic filter utilized hanging textile sheets. The prior art textile sheets were made of polypropylene and had a water holding capacity of 73%, an open area of 85.6% and a surface area of 4,381 ft2/ft3. The characteristics were chosen to mimic the characteristics of sand, which is a conventional filter media. It was believed that the textile filter media should have a maximum surface area and small pores (relatively low open area), since these are characteristics of fine grained sand filter media. In addition, it was thought that the filter media should have a high water holding capacity. It was believed that by increasing the residence time of the water in the filter media (by choosing a relatively high water holding capacity), the filter media would provide improved treatment of the septic tank effluent. The sheets were horizontally spaced apart to provide a gap between adjacent sheets of {fraction (5/16)}ths of an inch.
However, the present inventors found that the prior art textile material having a high surface area (4,331 ft2/ft3), a relatively low open area (85.6%) and high water holding capacity (73%) failed to provide the desired treatment. The aerobic filter did not perform well, as the textile filter media suffered from excessive biological growth and became clogged. The textile sheet also experienced hydraulic sheeting and anoxic conditions. In addition, the hanging sheets were subject to hydraulic load shorts through the gap between the sheets, resulting in poor effluent quality.
What is desired is an aerobic filter that is capable of providing high quality effluent in a relatively small space, and that supports an aerobic environment conducive to biological growth to provide nitrification of the septic tank effluent.
It is one object of the present invention to maximize the quality of septic tank effluent with respect to its ammonia nitrogen levels by providing an aerobic filter having a textile filter media that provides a superior aerobic environment for biological growth in order to provide nitrification.
It is a separate object of the present invention, independent of the previous object, to provide an aerobic filter comprised of a plurality of hanging sheets, in which the spacing between individual sheets is optimized for treatment of effluent.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.